On Tue, 20 Oct 2009 23:23 -0400, "cogoman" <[email protected]> wrote: > Thanks to Sven Wesley I was impressed by the AMT102-V Versapak > quadrature encoder available at Digi-Key. With 2048 PPR, which could > get 8192 quadrature increments it could give me a possible solution to a > router axis drive. Using skateboard bearings to support a 5/16" rod, a > DC motor could drive the rod with O-rings and pulleys. A fine steel > stranded cable wrapped around the rod many times would release about 1" > of cable per revolution. With 8192 steps per revolution, there would be > about 250u inches per encoder step. > > This caused me to wonder... How many encoder pulses per inch does a > professional CNC machine use.
Define "machine". Routers and mills are completely different machines in terms of accuracy, resolution, and rigidity. A commercial milling machine probably has 100,000 counts per inch or even more. But that is overkill for a router. If you have 10,000 counts per inch on a router you should be fine, and even 2-3000 would be OK. At a minimum, I would want to have at least 2-3 encoder counts over the tightest tolerances I expect to hold. It would be much better to have 10 counts. But you need to have a realistic idea of what tolerances you expect to hold. A large router cutting wood is NOT going to hold 0.001", no matter how good your encoder is. For starters, wood expands and contracts with changing moisture levels.... Consider just a couple other error sources: 1) thermal - if this is a router, I'm gonna assume that the long axis is at least 48". Wikipedia has a nice table of thermal expansion coefficients that says carbon steel expands by 10.8 parts per million per degree C. Over a 48" distance, your ballscrew (or steel cable) will change by half-a-thou for every degree of temperature change. 2) stretch - the tensile modulus of steel is 30,000,000 psi. If you use a 1/8" diameter steel cable, it has an area of 0.012 square inches. That means a 48" length of cable will stretch 0.00013" for every pound of tension. If you have 20 lbs of cutting force the cable will stretch by 0.0026". Regards, John Kasunich -- John Kasunich [email protected] ------------------------------------------------------------------------------ Come build with us! The BlackBerry(R) Developer Conference in SF, CA is the only developer event you need to attend this year. Jumpstart your developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now! http://p.sf.net/sfu/devconference _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
